More requirements for the capacity of a transmission network are brought forward by ever-increasing video services. A main high-end line bearer rate of a current optical transport network is 10 Gb/s, and the high-end line bearer rate of a next generation optical network is 40 Gb/s or higher than 40 Gb/s. In addition, the capacity of Ethernet services is growing at a speed of ten times, and requirements for a bearer bandwidth also increase with a factor of ten.
An optical line modulation format is critical to an optical transmission system, and a selection of the format directly affects transmission performance, spectral efficiency, non-linear tolerance, and dispersion tolerance of the system. FIG. 1 is a schematic diagram of generating a Differential Quadrature Phase Shift Keying (DQPSK) in the prior art. Service data to be transmitted is processed by a time division demultiplexer, and two paths of sub-service data signals are generated and are respectively input to a precoding unit to be precoded, thereby obtaining an I signal and a Q signal. The I signal and the Q signal are respectively input to two arms of a double-parallel modulator. A Laser Diode (LD) is used to provide the double-parallel modulator with an optical signal. The optical signal provided by the LD is modulated by the double-parallel modulator, and then a DQPSK optical signal carrying the service data is output. FIG. 2 is a schematic pulse diagram of a DQPSK optical signal in the prior art. The service data is represented by a phase difference between two adjacent pulses of the DQPSK optical signal. For example, information is carried through a phase change between a pulse 6 and a pulse 5. Subscripts in FIG. 2 are pulse sequences, and π/4 and the like are phase information of the pulses. FIG. 3 is a schematic constellation diagram of a DQPSK optical signal in the prior art. Because a quadrature modulation technology is adopted for the DQPSK optical signal, the constellation diagram has four constellation points 1, 2, 3, and 4.
The inventors find during the research that, the constellation diagram of the DQPSK signal in the prior art only has four constellation points, each of which can only carry information of two bytes, while growing transmission services such as video and data requirement for modulation format with higher spectral efficiency. Therefore, the DQPSK signal in the prior art cannot bear and transmit information at a higher rate. In addition, because polarization between adjacent code elements of the DQPSK signal in the prior art is the same, an interference effect cannot be avoided, and the non-linear effect is strong during transmission.